The present invention relates to vacuum cleaners and particularly, but not exclusively, vacuum cleaners for domestic use.
Typically vacuum cleaners in the art operate by creating a pressure difference by means of a rotating fan or impeller powered by an electric motor. The positive pressure is vented to atmosphere through a dispersion system, whilst the negative pressure is directed toward the end of a wand or tube that interfaces with the ground/object to be cleaned. The resulting flow of air through the vacuum cleaner passes through a dust trapping chamber either in the form of a porous bag having a porosity small enough to retain particles, or via a cyclonic chamber that catches the particles by virtue of centrifugal action acting on the swirling particles. The airflow continues from the top of the cyclonic chamber, or via an enclosing chamber in the case of a porous bag, to an exit filter before arriving at atmosphere in the form of a diffuse exhaust.
Various attempts over the years have been made to improve performance, for example by improving the efficiency of the impeller fan, by increasing the speed of the impeller fan, by carefully engineering the impeller to be a type of spiral form multi-blade turbine, by the use of cyclone technology where the air is directed to swirl so as to create a centrifugal component to the flow and to allow velocity increase by progressive reduction of swirl flow diameter.
Dust extraction in the present machines is principally a function of vacuum or negative pressure performance. This performance is in turn mostly dependent upon impeller design and efficiency. It is typically the case that impeller efficiency will be below 50%. However some recent turbine type impellers are purported to be as high as 70% efficiency. Total system performance is further reduced by factors such as loss through the wand system, and brush head design with respect to how air flow is constrained from atmosphere to the vacuum entry zone of the head.
With bag-type filters a small porosity will trap most particles, but too small a porosity results in flow constraints and large pressure drop across the bag. Thus there is a compromise that requires small particles to be let through the filter system. Cyclonic systems capture the primary dust particles by swirling centrifugal force acting on the particles causing them to be thrown against the container wall. Air flow from the container is filtered through a secondary system which again must have porosity to allow air through and so has to have a pathway for particles. Thus neither filtration technique is ideal or close to perfect and in both bag and cyclone systems the exhaust air flow by definition contains particles of dust and/or microbes that are too small for the filter systems to capture.
Problems are also associated with the ease of emptying dust collectors. Bags must be removed, unsealed and tipped into a waste bin. Often the bag is non-reusable and the whole bag is disposed of when full. In cyclonic system emptying typically has a removable vessel that is simply tipped up to dispose of the contents—at least this is the theory. In practice the vessel is often constructed of or coated with plastic material that becomes statically charged due to the movement and consequent friction of the particles of dust with the vessel inner surfaces. This triboelectric effect causes the particles to stick to the surfaces of the vessel and thus become difficult to empty.
Finally, the process of moving air at high velocities over insulate surfaces results in charge build up and substantial positive ionisation of the exhaust air. In particular this is an issue with high velocity cyclone type systems where the triboelectric effect is amplified by particle velocity and friction. The electric motor causes magnetically induced positive ionization whilst universal motors are more of an issue with brush sparking adding to the ionization effect and producing a fire hazard. The positive ionization is a known health hazard at worst and at best is believed to contribute to fuzziness of thought and higher propensity to illness.